Plating of Entamoeba histolytica and E. invadens are cloned colonies or as lawns of amebae was achieved by use of a double layer agar technique and incubation of the plates in a microaerophilic atmosphere for the former species and an anaerobic atmosphere for the latter. Cloning efficiencies were high. Strains exhibited distinct colony morphology, a factor possibly useful in classification. A lipoprotein-cholesterol fraction of bovine serum replaced whole serum in the axenic cultivation of four Entamoeba, E. histolytica, E. histolytica-like Laredo amebae, E. invadens and E. moshvoskii. Concanavalin A induced agglutination and the number of binding sites for the lectin on the surface of E. histolytica showed no correlation with the virulence of the strains examined. Moreover, SDS-PAGE, Southern blot studies showed no major qualitative differences in the membrane components of eight strains of the amebae. A putative restriction map for the ribosomal cistron of E. histolytica has been derived by probing enzyme digested amebic DNA with the cloned ribosomal cistron of Plasmodium lophurae. Ribosomal genes of E. histolytica appear to be higly repeated and in tandem array. The ribosomal cistron is at least 20,000 base pairs in length, and there appears to be large amounts of transcribed and non-transcribed spacer regions, excellent candidates for diagnostic probes. Extensive trials to find tubulin hybridizing sequences in E. histolytica restriction cut DNA using alpha- and beta-tubulin probes of Leishmania enrietti have been unsuccessful and suggest the absence of trubulin genes in this ameba. Suckling mice were used to advantage to obtain axenic cultures of several new strains of Giardia intestinalis from humans. Mice infected with cysts derived from feces served as sources of trophozoites which are more amenable to cultivation than the cyst stage.